1. Technical Field of the Invention
The present invention relates to methods and apparatus for measuring an aspherical shape, and methods for measuring an optical element using the methods and the apparatus.
2. Prior Art
An interferometer is often used to measure the shape of a plane or a sphere because two-dimensional data can be collected within a short time. When a general plane or a sphere is measured using an interferometer, a reference surface standard with infinitively small dimensional errors for the plane or the sphere is required. More explicitly, when measuring the shape of a sphere, spherical surface standards with several radii are prepared, an interference band is formed using reference light from the spherical surface standard and measurement light from a surface under measurement, and the shape of the surface under measurement is determined with respect to the spherical surface standard.
To produce an aspherical optical element such as an aspherical lens and an aspherical reflecting mirror with high precision and high efficiency, the use of the aforementioned interferometer is strongly required for interference of the shape of the aspherical surface. However, when a conventional spherical interference means is applied to an aspherical surface, object surfaces must be very limited, simple shapes such as an aspherical surface with only a slight deviation from a genuine spherical surface or a cylindrical surface; the former is measured using a spherical surface standard and CGH, and the latter using a cylindrical lens and HOE.
In other words, when an aspherical surface is measured with an interferometer using a conventional spherical surface standard, no interference band is produced if the, shape error from a true sphere is more than 10 times the wavelength (for instance, about 10 xcexcm or more), so an interference band can be observed only over an extremely narrow range, therefore, the entire aspherical surface cannot be measured within a short time.
Conversely, this problem can be solved in principle by manufacturing an aspherical standard with extremely small errors in shape from a surface under measurement, like a spherical shape. However, compared to a spherical surface standard, an aspherical surface standard is very difficult to manufacture. If such a standard can be manufactured, an aspherical surface under measurement can also be machined with a high accuracy. In addition, when the absolute shape of an aspherical surface standard is evaluated under an interference measurement, another aspherical surface standard with a higher accuracy must be used. Moreover, the aspherical surface of an object under measurement for substantially every radius of curvature cannot be measured using spherical surface standards with several radii of curvature, each of which has a spherical surface. Therefore, one aspherical surface standard is required with an aspherical surface.
Consequently, it is essentially impossible to measure an aspherical shape using an interferometer and a aspherical surface standard, except for very limited cases such as when a large lot of identical aspherical surfaces are measured.
The present invention solves the problems mentioned above. That is, an object of the present invention is to provide methods and apparatus for measuring an aspherical shape within a short time without using an aspherical surface standard and methods for manufacturing optical elements using the aforementioned methods and apparatus.
The present invention provides methods for measuring an aspherical shape, whereby an aspherical reference surface (2) is manufactured with such an accuracy of shape that an interference band is produced according to the aspherical shape of a surface (1) to be measured, thereby an aspherical wave front (3) is formed, and an aspherical surface with a large area is measured using an interferometer within a short time.
According to the above-mentioned methods, the aspherical reference surface (2) is manufactured with such an accuracy of shape that an interference band appears. Therefore, compared to a spherical surface standard or an idealistic aspherical surface standard, machining accuracy can be lower with a greater degree of freedom in selecting a material, so that the aspherical reference surface can be provided easily. The aforementioned accuracy of shape to such a degree that an interference band is seen, means that machining tolerance can be no more than several times as large as the wavelength of light used for the interference measurement (for instance, laser light). Although the accuracy of the shape is inferior to that of the standard, the accuracy of the shape is maintained at such a level that an interference band can be observed. Therefore, a wide range of aspherical surface shape can be measured from interference simultaneously and quickly.
The accuracy of the shape of the above-mentioned aspherical reference surface (2) is measured beforehand, and added to the results of the aforementioned measurement, thus the shape of the surface being measured is determined. The accuracy of a shape such as the aspherical reference surface can be measured highly precisely using another high-accuracy measuring instrument (for instance, needle-contact-type shape measuring apparatus etc.). Therefore, by measuring the shape in advance, a large area of the aspherical shape can be measured using an interferometer, within a short time, by adding the results of the above-mentioned measurement.
The above-mentioned aspherical reference surface (2) is manufactured by fly-cutting or ELID-grinding (electrolytic in-process dressing grinding) . When the aspherical reference surface is a reflecting surface, a high-accuracy mirror surface can be easily manufactured by fly-cutting a metal surface. Even when the aspherical reference surface is a reflecting surface or a transparent surface such as a lens, it can be ground at a high accuracy by ELID-grinding.
In addition, the present invention provides an apparatus for measuring the shape of an aspherical surface with an aspherical surface optical element (10) having an aspherical reference surface (2) that matches the aspherical surface shape of surface (1) under measurement, thereby an interference band is formed by light reflected from the aspherical surface of an aspherical wave front (3) produced by the aforementioned aspherical reference surface and predetermined reference light, and the shape of the aspherical surface is interference measured.
According to this configuration, an aspherical wave front (3) is formed by the aspherical optical element (10) with the aspherical reference surface (2), and by achieving the interference band using light reflected from the aspherical surface and the predetermined reference light, the shape of the aspherical surface can be measured using an interferometer.
The above-mentioned aspherical optical element (10). is an aspherical reflecting mirror whose shape accuracy is such that the interference band appears, and parallel light is reflected in the normal direction of the surface under measurement. In this configuration, the aspherical reflecting mirror is manufactured with such a shape accuracy that an interference band appears, parallel light is reflected in the normal direction of the surface under measurement, the interference band is formed by reflected light and predetermined reference light, thereby the shape of the aspherical surface can be measured using an interferometer. In addition, an aspherical surface light element can be provided easily, because such shape accuracy as that producing the interference band is lower than those of a spherical surface standard or an ideal aspherical surface standard, and the material of the reflecting mirror can be chosen from among a wide range of materials.
Furthermore, according to the present invention, a plurality of aspherical reference surfaces (2) are manufactured using the aforementioned methods or apparatus, thus a plurality of aspherical wave fronts (3) are formed, and a wide aspherical area is interference measured within a short period. Using these methods, particularly when the surface (1) under measurement is large, the aspherical reference surface (2) required can be made compact while maintaining its accuracy sufficiently high, as well as measuring the large surface under measurement simultaneously and quickly, using an interferometer.
Other objects and advantages of the present invention are revealed by the following explanations referring to the attached drawings.